Well, bummer. Car runs better with new cats, but my mileage actually went DOWN to 17 mpg. What gives? Anybody got an idea what might be causing this? (it was 24 mpg, then after CEL for cats it went down to 18 mpg) Do I need a tune up after installing new cats? The car runs great.

after that I would suspect oxy sensors.

did you happen to reset the ecu?

it prolly adjusted itself to work with the old plugged cats

 

also do you know if any of the cat guts broke off? there maybe some stuck in the muffler

 

 

mmm roasted cat guts.. lol